Patent GB-973695-A (GB)
Title: Improvements in or relating to signalling systems
IPC: B61L25/04
Filed: 1960-10-31
Abstract: 973,695. Track apparatus &c. GENERAL PRECISION Inc. Oct. 31, 1960 [Nov. 4, 1959], No. 37350/60. Heading B7N. [Also in Divisions H3 and H4] Relates to a system of the type described in Specification 917,755 wherein a plurality of different frequencies are transmitted as modulation of a carrier by an interrogator equipment and a responder retransmits a selection of the frequencies on a different carrier frequency, a receiver and decoder for the response signal including means for ensuring that the signal passed for decoding is in correct phase relationship with a comparison signal. The system is applicable to the identification of railway cars as they pass along a track. As shown, in Fig. 1, at the interrogator 10 a plurality of audio-frequency oscillators 11 to 15 through a modulation circuit 17 amplitude modulate a carrier generated at 16, the resulting signal being supplied via a gain control device 18 and amplifier 19 to an inductor 20 which may be mounted between the rails of a track. Railway cars each carry a responder unit 22 which as it passes the inductor 20 provides a response signal unique to that responder. At a typical responder unit, Fig. 2, the interrogating signal causes a signal to be induced across a circuit 56 which is demodulated by a diode X1 and capacitor C2 and the D.C. component plus the audio-frequencies are supplied via a circuit 53 rejecting one of the frequencies to power a carrier generating oscillator consisting of a transistor T1 and tuned circuit 55. The oscillator is also modulated by the frequencies passed by the circuit 53. Resistor R100 and capacitor C100 are included to prevent clipping of the modulating frequencies. The impedance of the capacitor C100 at the lowest modulating frequency is much lower than the resistance of R100 and the resistance of R100 is made equal to the D.C. load resistance of the detector circuit. The response signal is received by a coil 60, Fig. 1, and after amplification is supplied to a synchronous detector 61 provided with reference oscillations at carrier frequency from a generator 62 and the output of the demodulator is supplied via a low pass filter 63 and amplifier 64 to filters selecting the audio frequencies and operating a register. Alternatively, further synchronous detectors receiving reference signals direct from the oscillators 11 to 15 may be used to select the audio frequencies, Fig. 5 (not shown). One of the audio frequencies transmitted by the interrogator is always retained in the response signal and is supplied from the amplifier 64 to a phase comparator 69. This audio frequency (from the oscillator 15) is also supplied direct to the receiver and after a 90 degrees phase shift at 68 is used as a reference signal for the phase detector 69. If the local oscillator 62 is phase-locked with the incoming signal from coil 60 the two signals supplied to the phase detector 69 will be in quadrature and there will be no output from the detector 69 but if the local oscillator frequency should vary, a change in phase between the two input signals to the detector 69 will occur and an output voltage will be produced to correct the frequency of the oscillator 62 via a circuit 62a. In an alternative arrangement, Fig. 3 (not shown), it is not necessary to reserve one of the audiofrequencies as a reference and a reference frequency direct from the interrogator is not required. The response signal is supplied to two synchronous detectors one of which is supplied with a locally generated signal at carrier frequency, and the other with the locally generated signal phase-shifted by 90 degrees and thus providing no output when the local oscillator is correctly phased. The outputs of the two detectors, after filtering and amplification, are supplied to a phase comparator to control an automatic frequency control circuit of the local oscillator. In a further embodiment, Fig. 4 (not shown) at the responder the received carrier frequency is multiplied and the oscillator is controlled to transmit at this frequency. The carrier frequency is also supplied direct from the interrogator to the receiver and after the same multiplication used as the reference frequency for the synchronous detector, thus dispensing with a reference oscillator at the receiver.